Objective: To provide genetic counseling and prenatal diagnosis for a fetus with mosaic trisomy 20.
Methods: Chromosomal karyotyping, chromosomal microarray analysis (CMA) and fluorescence in situ hybridization (FISH) were carried out for a pregnant woman with advanced maternal age.
Results: The karyotype of amniotic fluid sample was 47,XN,+20, whilst the result of CMA was normal. To verify this discrepancy, CMA was performed again with the cultured amniotic fluid, which yielded a result of 47,XN,+20. FISH assay of the amniotic fluid sample was nuc ish(D20Z1)×3[11]/(D20Z1)×2[89], which indicated that about 11% of fetal cells were trisomy 20. After the fetus was born, the karyotype of peripheral blood sample was normal.
Conclusion: The amniotic fluid sample might be mosaic trisomy 20, and a dominant growth of 47,XN,+20 cells had occurred during the culture process, resulting in alteration of amniotic fluid cell composition. Mosaic trisomy 20 indicated by FISH may be attributed to confined placental mosaicism or somatic mosaicism of trisomy 20.